The folowing quote is from a Wikpedia article https://en.wikipedia.org/wiki/Neutrino . "From cosmological arguments, relic background neutrinos are estimated to have density of 56 of each type per cubic centimeter and temperature 1.9 K (1.7×10−4 eV) if they are massless, much colder if their mass exceeds 0.001 eV." There is no reference given for the "facts" presented in the quote. I entered a request for such a reference on the Talk page, but I expect it will take some time for this reference to become included. Can anyone help me find a reference about this that will include an explanation about assumptions and the method used to calculate the numbers?